Method for Making Silicon-Containing Products

ABSTRACT

A method for producing carbon-silica products from silica-containing plant matter such as rice hulls or straw by leaching with sulfuric acid to remove non-silica minerals and metal while adjusting the mole ratio of fixed carbon to silica in the resultant product. The carbon and silica are intimately mixed on a micron or submicron scale and are characterized by high purity and reactivity, small particle size, high porosity, and contain volatile carbon that can be used as a source of energy for the production of silicon-containing products from the carbon-silica products. High purity silicon-containing products made from the carbon-silica products of the invention are also disclosed.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to various silicon-containing productsand to methods for making such products from starting materials obtainedfrom plant matter or processed plant matter having a significant silicacontent, most preferably rice hulls and rice straw. One aspect of theinvention relates to carbon-silica products made by a leaching processwherein the starting materials are treated with a sulfuric acidsolution. Another aspect of the invention relates to chemical andthermal methods for removing and recovering volatile carbon fromcompositions made by the leaching process, and to the resultantdevolatilized carbon-silica products. Another aspect of the inventionrelates to silicon-containing products made from devolatilizedcarbon-silica products using a carbothermal process.

2. Description of Related Art

The unique performance properties of high purity silicon have made thedevelopment of the semiconductor industry possible and are important tothe rapidly growing photovoltaic industry. Other well knownsilicon-containing materials include, for example, silicon carbide andsilicon nitride, which, in certain forms, are used to produce highperformance ceramics and high performance composites. These, and othersilicon-containing materials, are used in a variety of applicationsincluding electronics, defense, automotive, aerospace, industrial wearparts, advanced glasses, and in chemical and environmental products.

The photovoltaic industry is growing at a rapid pace, but the cost ofsilicon is one of the deterrents to even faster growth and to theproduction of bulk power using photovoltaic panels. Due to a lack ofcost-effective processes for making solar-grade silicon, a majority ofsolar cells are presently made from the purer and more costlysemiconductor-grade silicon. Likewise, the demand for high performanceceramics and composites is growing, but the promise of these industriesis hampered by the high cost of materials such as silicon carbide andsilicon nitride.

The production of essentially all materials and products containingsilicon involves the reaction of carbon and silica (SiO₂) at a very hightemperature, often referred to as a carbothermal reduction. The carbon“pulls” oxygen atoms off the silica and the resulting carbon monoxideexits the reactor as a gas, leaving behind the silicon product. If thedesired product is silicon by itself, then the molar ratio of fixedcarbon to silica for this reaction should be 2:1 as shown below:

If the desired product is silicon carbide (SiC), the molar ratio for thereaction should be approximately 3:1, as shown by the following formula:

Other Si-based products, such as ferrosilicon (FeSi) and silicon nitride(Si₃N₄) and silicon tetrachloride (SiCl₄) can be produced using the samereactions between carbon and silica. For example, to make siliconnitride, two moles of carbon are used per mole of silica, while thereaction is carried out under a nitrogen-containing atmosphere.

The standard commercial process for producing silicon involves themixing of a carbon source such as coke with crystalline silica, i.e.sand or quartz, in a predetermined ratio and reacting this mixture toproduce metallurgical grade silicon. Because the coal and sand particlesare large and not very porous, with limited contact and availablesurface area, the rate of reaction using the conventional feedstocks isslow, typically taking more than a day to complete. As a result, theenergy requirements for this carbothermal process are high, and thesilicon produced is often less than 99% pure. Silicon produced in thismanner is suitable for applications in the aluminum and certain chemicalindustries, but is not of adequate purity for such applications as thesemiconductor and photovoltaic industries. Very expensive andcomplicated processes have previously been needed to upgrade themetallurgical silicon to photovoltaic and semiconductor grade materials.

Similarly, silicon carbide produced using the slow and energy-intensiveAcheson process is costly and is limited to use in the metallurgical,refractory and abrasives industries and in other applications that donot require high purity, small particle size, and/or whiskers or fibers.The production of high-end silicon carbide powders has previouslyrequired the use of processes that are even more expensive andcomplicated.

Silicon nitride can be produced through a variety of processes and isoften made commercially using expensive starting materials such assilicon tetrachloride. The resultant powder, although commerciallydesirable, is too expensive for use in all but a few high-endapplications.

During the past 20 years, several researchers have investigated the useof rice plants as a source of silica for industrial products. All plantmatter contains significant amounts of carbon and many types of plantscontain silica. The rice plant is perhaps one of the most unique becauseof its high concentrations of silica. Whereas the mineral content ofmost plants is, for example, about 1-2%, the rice plant typically has amineral content of about 11-23%. More significantly, about 75-95% of themineral content of the rice plant is silica. Rice straw contains about11% silica and rice hulls typically contain about 15-23% silica.Although rice is one of the most abundant crops grown worldwide, to datethere has been little practical use for rice hulls and rice straw, whichconstitute a considerable portion of the rice plant.

Rice hulls are the natural sheaths that form on rice grains during theirgrowth. They are removed during the refining of rice and are a waste orlow value by-product of the rice milling industry. Rice straw consistsof stem, leaf sheathes, leaf blades and the remains of the panicle afterharvesting. Generally, the amount of rice straw obtained from riceplants is at least equal to the rough yield of rice harvested. Becauseof their high silica content, these materials have little value ascomponents of animal feeds. Because rice hulls and rice straw have arelatively large amount of potassium that interacts with the silica atcombustion temperatures to produce boiler slag and deposits, and have alarge fraction of noncombustible ash, they are similarly viewed as beinga poor fuel source. If rice hulls and rice straw are burned as fuel, therelatively high ash content of both rice hulls and rice straw requiresspecial handling equipment. For these reasons, rice hulls are frequentlydeposited in landfills and rice straw is usually burned in the fields.Thus, rice hulls and rice straw have little-to-no commercial value andhave historically presented a disposal problem.

Nevertheless, rice hulls and rice straw continue to be an attractivesource of silica because of the high silica content and low cost. Mostof the organic material in rice hulls can be removed by combustion. Theash produced from such combustion processes comprises up to about 95%silica, but still contains non-silica mineral impurities. Severalinvestigators have studied the removal of non-silica minerals from ricehulls in order to create value-added products from the “purified” hulls.

L. P. Hunt, J. P. Dismukes, J. A. Amick, “Rice Hulls as a Raw Materialfor Producing Silicon,” J. Electrochem. Soc., 131(7), 1984, investigatedthe potential use of rice hulls for producing silicon pure enough forfabrication into solar cells and low enough in cost so that photovoltaicenergy could be more cost-competitive with conventional energy sources.Following grinding of raw rice hulls to −20 to +80 mesh, washing anddrying, two samples of the dried hulls were leached for 15 minutes underboiling conditions with HCl:deionized water at 1:3 and 1:10,respectively. Leaching with 1:10 acid solution was found to be just aseffective as 1:3 acid solution. However, leaching a sample with theweaker acid solution for 5 hours at 50° C. did not reduce impurityconcentrations to levels as low as those attained under boilingconditions. Three rice hull samples from different sources were acidleached for 15 minutes in boiling 1:10 HCl acid solution. Concentrationsof calcium, potassium, magnesium and manganese were reduced by factorsof 40-100 times (97.5 to 99% removal). Sulfur concentration was reducedby a factor of 8 (87.5% removal); sodium and phosphorous concentrationswere reduced by a factor of about 3 (67% removal); boron, aluminum andiron concentrations were not reduced. The investigators reported thatraw rice hulls have a total non-silica mineral impurity concentrationabout 30 times greater than that of leached hulls (overall 96.7%), andprojected that an acid-leached and coked product is of interest as a rawmaterial for the production of solar-grade silicon. However, it wasnoted that since the phosphorous/boron ratio exceeded 10, eventualfabrication of solar cells would require a different process tosubstantially reduce the concentration of phosphorous, which wasreported at 40 ppm (averaged).

The effect of porosity on encouraging the production of silicon carbidewhiskers as well as the importance of porosity in removing mineralimpurities is disclosed in U.S. Pat. No. 4,504,453 (1985) to Tanaka.

M Patel, A. Karera and P. Prasanna, “Effect of thermal and chemicaltreatments on carbon and silica contents in rice husk,” J Mater Sci, 22(7), 1987, report subjecting rice husk samples obtained from thevicinity of Bhopal, India, to treatment with laboratory gradehydrochloric (4 to 12 N), sulfuric (2 N) or nitric (8 N) acid for 2 to 6hours at 100° C. The authors concluded that SiO₂ of 99% purity (10,000ppm of non-silica minerals, which is a high impurity level for highpurity applications) can be produced from rice husk treated with HCl,followed by carbonization at temperatures below 700° C. to avoid anytransformation of amorphous to crystalline form, and that the puritycannot be increased above 99% because the remaining 1% may be metaloxides insoluble in acid. They also reported that reflux reactions aredifficult to carry out in sulfuric or nitric acid, and therefore onlylimited experiments were performed. The authors also reported that theywere able to achieve a carbon:silica ratio of 2:1 by partial coking orrice hulls. However, this was a mass ratio that corresponds to a moleratio of 10:1, whereas the desired molar ratios are in the range of 3:1to 2:1.

A. Chakraverty, P. Mishra and H. D. Banerjee, “Investigation ofcombustion of raw and acid-leached rice husk for production of pureamorphous white silica,” J. Mater Sci, 23(1), 1988, disclosed millingcleaned and dried rice husks to a particle size of about 40 mesh,leaching in hydrochloric (1 N, 3 N, 5 N and 11.3 N), sulfuric (1N, 4.5N, 9 N and 18 N) and nitric (4.5 N, 9N and 18N) acid at 50° C. for 2hours, washing with distilled water, and drying. The acid-leached huskswere then combusted at temperatures ranging from 500 to 700° C. Acidtreatment with sulfuric acid was less effective than leaching witheither hydrochloric acid or nitric acid at comparable concentrations forreducing the concentrations of oxides of sodium, potassium, calcium,iron, magnesium, manganese, zinc and copper. The overall metallicimpurity level (reported as oxides) ranged from 300-747 ppm for samplestreated with hydrochloric acid, from 496-688 ppm for samples treatedwith nitric acid, and from 3534-4483 ppm for samples treated withsulfuric acid. Even the best performance, achieved using 11.3 N HCl,failed to produce a 99% reduction in the identified non-silicaimpurities. No data was presented regarding phosphorous levels eitherbefore or after treatment.

R. Conradt, P. Pimkhaokham and U. Leela-Adisorn, “Nano-structured silicafrom rice husk”, J. of Non-Crystalline Solids, 145 (1992) 75-79, reportthe acid leaching of washed That rice husk by reflux boiling in 2.4molar hydrochloric acid or 3.6 molar sulfuric acid for 3 hours at aratio of 100 g. husk/liter. The leached husks were subsequentlyincinerated at 600° C. and characterized. The investigators concludedthat omission of acid pre-treatment yielded a considerably reducedsurface area in the incinerated products. Specific surface areas of180-250 sq. m/g. are reported for the silica prepared from rice husks.The best non-silica mineral level achieved in the ash obtained from ricehulls pretreated with HCl was 6500 ppm. The best non-silica mineralpurity level in the ash obtained from rice hulls pretreated withsulfuric acid was 10,000 ppm.

I. A. Rahman, “Preparation of Si₃N₄ by Carbothermal Reduction ofDigested Rice Husk,” Ceramics Int'l (1994), investigated the productionof carbon and silica through digestion of rice husk using nitric acid.About 30 g of prewashed rice husk was digested in 300 ml of nitric acidat 60° C. The concentration of acid was varied from 10M to 14M. Duringdigestion (for up to 7 hrs. with continuous stirring), the temperaturewas carefully controlled. The digested husk was then filtered and washedwith distilled water until neutral. The overall non-silica mineral level(reported as oxides) in the digested husks was reported to be 2,500 ppm.The digested husk was pyrolysed at 800° C. to a constant weight under aflow of argon gas. The carbon content of pyrolysed digested husk wasdetermined by heating at 700° C. in air for one hour. Rahman reportedthat after more than 3 hours digestion in 12M nitric acid, the digestionproduct obtained was in the stoichiometric ratio of 2C/SiO₂ and thathigher concentrations could not produce the desired ratio. Lowerconcentrations required a longer time to produce a suitablecarbon-silica mixture. The weight loss after heating was considered tobe the amount of carbon present in the pyrolysed digested husk, and theresidue was considered to be pure silica. Next, the pyrolysed powder wasnitrided by heating in a furnace in a controlled nitrogen atmosphere,raising the temperature gradually to 1430° C., after which the productswere burned in a muffle furnace at about 700° C. for 30 min. to removeany excess carbon. The most important factor contributing to thecompleteness of reaction was found to be the homogeneity of mixing, andthe use of rice husk was said to be an advantage, as the silica andcarbon are naturally mixed.

C. Real, M. Alcala and J. Criado, “Preparation of Silica from RiceHusks,” J. Am. Ceram. Soc., 79(8) 1996, investigated a procedure forobtaining pure silica gel with a high specific surface area from ricehusks and reported that silica with >99% purity can be obtained byburning rice husks at 600° C. under inert atmosphere, followed by thecombustion of the residual carbon under oxygen atmosphere at the sametemperature, provided that either the husks used as raw material or thesilica obtained as final product have been leached previously in boiling10% hydrochloric acid solution for 2 hrs. However, the yield of a silicagel with a high specific surface area and a homogeneous distribution ofnanometric particles requires either the preliminary leaching of therice husks with diluted hydrochloric acid or its washing with boilingwater to remove the K⁺ cations.

R. V. Krishnarao and J. Subrahmanyam, “Formation of SiC from Rice HuskSilica-Carbon Black Mixture: Effect of Rapid Heating,” Ceramics Int'l,22 (1996) 489-492, demonstrated that porosity is important for theproduction of silicon carbide whisker.

N. Yalcin and V. Sevinc, “Studies on silica obtained from rice husk,”Ceramics Int'l 27 (2001) 219-224, report leaching of washed and driedrice husks by reflux boiling in 3% (v/v) HCl or in 10% (v/v) sulfuricacid for 2 hours at a ratio of 50 g. husk/l, or by leaching with 3%(v/v) NaOH solution for 24 h at room temperature at a ratio of 50 ghusk/l. After leaching the husk was thoroughly washed with distilledwater, dried in an air oven at 110° C. and then burned in a mufflefurnace at 600° C. by four different methods. The investigators foundthat the silica content of rice husk ashes was strongly dependent uponthe type of chemical used for leaching the rice husks prior toincineration. Reported SiO₂ content (wt %) was 99.60±0.05 (4000 ppm) forthe ash samples where the husks were pre-leached with 10% (v/v) sulfuricacid (v/v) for 2 hr. at the boiling point, and 99.66±0.02 (3400 ppm) forthe ash samples where the husks were both pre- and post-leached with 3%HCl (v/v) for 2 hr. at the boiling point. The sample leached withsulfuric acid before incineration exhibited a BET specific surface of282 m²/g.

Except for Hunt et al and Rahman et al., the foregoing references do notsuggest that the carbon found in rice hulls can be used as a source ofcarbon to be reacted with the silica in rice hulls to form desiredproducts. Instead, the rice hulls are burned to form ash, therebyremoving all of the carbon, after which the ash is treated with acid toremove minerals from the resulting silica. However, because the removalof K⁺ prior to heating the rice hulls allows the silica to maintain ahigher specific surface area and smaller particle size upon heating,Real et al. in 1996 demonstrated that it is advantageous to carry outacid leaching prior to using combustion to reduce the rice hulls tosilica. In either situation, the resulting silica is combined withcarbon or another reducing agent from another source in order to producethe desired silicon-containing product.

Rice hulls contain both fixed and volatile carbon. Fixed carbon isretained in the solids to high enough temperatures to react with silicato form products, while volatile carbon is volatilized at relatively lowtemperatures, making it unavailable to react with the silica. The moleratio of fixed carbon to silica in untreated rice hulls is about 4:1.Consequently, the mole ratio must be adjusted to the proper ratio forthe desired product (e.g., about 3:1 for SiC and about 2:1 for Si). Itis important to note that this is a mole ratio and is not a masspercentage ratio, as is sometimes used in the prior art. Even a masspercentage ratio as low as 1:1 correlates to a mole ratio of about 5:1,which is well above what is required in most carbothermal reactions.Hunt et al. referenced the use of pyrolysis followed by controlledcombustion with CO₂ to remove some of the fixed carbon after the mineralcontent had been previously reduced to moderate levels with HCl, whichlevels contained unacceptable amounts of phosphorous and no reduction ofiron. Rahman et al. used concentrated nitric acid, which is a strongoxidizing agent that disintegrates organic material in the husk, toremove carbon. However, the mineral purity of the ash from the treatedhusks was still 2500 ppm, with an iron content at 200 ppm. No data wasreported regarding phosphorous. As noted by Rahman et al., the highreactivity of the carbon after acid treatment, the retention of the highspecific area of the silica after acid treatment and the intimateassociation of the silica and carbon in acid treated rice hulls shouldenhance the rate of reaction between the silica and the carbon at hightemperatures.

To make materials that can cost effectively achieve the propertiesrequired for solar power and high performance materials, it is criticalto develop processes that can achieve adjusted mole ratios of intimatelyassociated fixed carbon and silica, and that can achieve very low levelsof non-silica minerals, particularly phosphorous and iron, from plantmaterial containing high levels of silica. These materials can then beused in less-expensive carbothermal processes to produce high-endsilicon, silicon carbide, silicon nitride, silicon tetrachloride andother silicon-based products at a much lower cost than the currentprocesses.

SUMMARY OF THE INVENTION

The present invention includes novel processes and intermediate productsthat can be used to produce high-performance, high purity end productssuch as silicon, silicon carbide, silicon nitride, silicon tetrachlorideand other high purity, silicon-containing products with lower energyrequirements and higher throughputs at a fraction of the costs incurredthrough the use of conventional processes. Naturally occurring plantmatter, processed plant matter, or mixtures thereof, containingsufficient quantities of both carbon and silica is leached in either asingle-stage or multi-stage process using a sulfuric acid solution at acontrolled temperature for a suitable time. The solid carbon-silicaproducts obtained from the leaching process contain a mole ratio offixed carbon to silica of at least about 1:1, very low levels ofminerals, and high porosity and reactivity. As used throughout thisSummary of the Invention, the Detailed Description of PreferredEmbodiments and the Claims, the terms “mineral” and “minerals” excludesilica and mean one or more of sodium (Na), potassium (K), magnesium(MG), calcium (Ca), manganese (Mn), iron (Fe), aluminum (Al), boron (B)and phosphorous (P), whether present alone or in combinations with eachother, or with other metals or atoms, such as oxygen. The fixed carbonand silica are intimately mixed on a micron or even submicron scale, andthe silica is present in an amorphous form with large specific surfacearea. The carbon-silica products of the invention are frangible andeasily reducible to a fine powder form by milling or other suitablemeans.

The carbon-silica products of the leaching process contain volatilecarbon that can be removed and recovered by further treatment with acid,or by heating to temperatures up to about 1000 deg C. The devolatilizedcarbon-silica products of the invention embody all the advantages of thecarbon-silica products mentioned above and also have low levels ofvolatile carbon. The devolatilized-carbon-silica products can easily bemilled to fine powder. Pellets can be formed from either thedevolatilized or undevolatilized carbon-silica products without addingexternal binders.

The carbon-silica products of the invention, whether or notdevolatilized, milled or pelletized, can be further reacted in acontrolled environment in a carbothermal reactor to produce desiredsilicon-containing products including but not limited to silicon,silicon carbide, and silicon nitride and silicon tetrachloride. Many ofthe products made according to the improved carbothermal process of theinvention are themselves believed to be novel because of their higherpurities and enhanced performance characteristics. The high reactivityof the carbon after acid treatment, the retention of the high specificsurface area of the silica after acid treatment and the intimateassociation of the silica and carbon in the carbon-silica products ofthe invention enhances the rate of reaction between the silica and thecarbon at high temperatures. During the carbothermal process, anyremaining volatile carbon can also be removed and recovered.

Because each particle of the carbon-silica products and devolatilizedcarbon-silica products of the invention embodies very small pieces ofcarbon and silica mixed together at a micron, submicron, or evennanometric scale, the contact points between the silica and the carbonare very large. Also, because the carbon and silica are amorphous andhave large surface areas, the material reacts very fast when producingsilicon-containing products. For example, reaction times of about 75minutes to produce silicon carbide are readily achievable, compared to aday or more when conventional coal, or coke, and sand are used as feedmaterials. This in turn means that less than about 1/20 of the energy isrequired to produce the same amount of product. Because energyconsumption is a major operating cost for the conventional carbothermalprocess, use of the process and materials disclosed herein will producesignificant savings in operating costs. This also means that a givenreactor can produce approximately 20 times as much silicon carbideproduct in a day as can be produced using a conventional carbothermalprocess.

According to one embodiment of the invention, a method is disclosed forleaching naturally occurring plant matter, processed plant matter, andmixtures of naturally occurring and processed plant matter, embodying atleast about 3 weight percent silica with sulfuric acid to producepreferred, high purity carbon-silica products. The use of rice hulls asthe plant matter is preferred.

According to another embodiment of the invention, a method is disclosedfor making carbon-silica products comprising the steps of comminutingplant matter comprising at least 3 weight percent silica to a particlesize ranging from about 10 microns to over 1 millimeter, and contactingand preferably mixing from about 1 to about 35 weight percent of thecomminuted plant matter with a sulfuric acid solution having an acidconcentration ranging from about 0.01 weight percent to about 30 weightpercent at a temperature ranging from about 10° C. to about 250° C. fora time ranging from about six seconds to about 48 hours. During theleaching process, minerals are leached out of the plant matter and theratio of fixed carbon to silica is preferably adjusted to a mole ratioof from at least about 1:1 up to about 5:1 or greater for use in someproducts.

According to another embodiment of the invention, a leaching method isdisclosed that comprises two or more stages of leaching. Followingapplication of the leaching method as described above, leaching of thewashed or unwashed material is repeated one or more additional times tofurther reduce the remaining mineral content and further adjust theratio of fixed carbon to silica. Using the multi-stage leaching methoddisclosed herein, fixed carbon-to-silica mole ratios as low as 1:1 orlower can be achieved.

According to another embodiment of the invention, a leaching method isdisclosed that is useful in either single or multiple stages forimproved removal of minerals from naturally occurring or processed plantmatter containing significant silica contents of greater than about 1weight percent and preferably greater than 3-5 weight percent or more.

According to another embodiment of the invention, a leaching method isdisclosed that is useful for reducing the amount of volatile carbon innaturally occurring or processed plant matter containing more than about3 weight percent silica.

According to another embodiment of the invention, a leaching method isdisclosed that is useful for reducing the amount of fixed carbon innaturally occurring or processed plant matter containing more than about3 weight percent silica.

According to another embodiment of the invention, a leaching method isdisclosed that is useful for converting volatile carbon into fixedcarbon in naturally occurring or processed plant matter containing morethan about 3 weight percent silica.

According to another embodiment of the invention, a leaching method isdisclosed that is useful for increasing the porosity of the carbon andsilica contained in naturally occurring or processed plant matter havingmore than 3 weight percent silica.

According to another embodiment of the invention, high puritycarbon-silica products are disclosed that comprise fixed carbon andsilica in a desired mole ratio and minerals at a level well below 0.1weight percent. The high purity carbon-silica products exhibitsignificantly improved porosity and reactivity, and are a preferredmaterial for use in making other silicon-containing products asdisclosed below.

According to another embodiment of the invention, carbon-silica productsare disclosed that have a mole ratio of fixed carbon to silica rangingfrom less than about 2:1 to greater than about 5:1, a total mineralcontent less than about 200 ppm, most preferably less than about 100ppm, a boron content less than 1 ppm, and a phosphorous content lessthan 4 ppm.

According to another embodiment of the invention, carbon-silica productsare disclosed in which the carbon and silica are intimately mixed at amicron, submicron or nanometric scale. These carbon-silica products canbe produced in the form of a fine powder or made into pellets withoutthe need for external binders, and are preferred for use in thesubsequent production of silicon-containing products including, but notlimited to, silicon carbide, silicon nitride, silicon, and silicontetrachloride.

According to another embodiment of the invention, carbon-silica productsare disclosed that comprise accessible internal void volume of fromabout 30% to about 90%, and most preferably greater than about 50%.

According to another embodiment of the invention, carbon-silica productsare disclosed that are frangible and easily reduced in size with minimalenergy consumption to powdered particles sizes in the fine (500 μm-50μm), very fine (50 μm-5 μm) and ultrafine (5 μm-submicron) categories.The average size and size distribution of the resulting particles iscontrolled by the process parameters as disclosed herein according tothe characteristics desired in the final silicon-containing products.

According to another embodiment of the invention, thermal and chemicaldevolatilization processes are disclosed that produce high purity,devolatilized carbon-silica products that exhibit significantly improvedporosity and reactivity, and are themselves an embodiment of theinvention. Both the devolatilized and undevolatilized products made bythe inventive method are preferred materials for use in making othersilicon-containing products, including, but not limited to, siliconcarbide, silicon nitride, silicon, and silicon tetrachloride.

According to another embodiment of the invention, a carbothermal processis disclosed that generally involves heating the carbon-silica productand/or the devolatilized carbon-silica product as disclosed herein in acarbothermal reactor to a very high temperature to initiate reduction ofthe silica to a silicon-containing product. The carbothermal reactordesirably heats the presently disclosed carbon-silica product to about1250° C. to about 2200° C., depending upon the silicon-containingproduct of interest. For silicon or silicon carbide, an inert atmosphereis used. For other products, different atmospheres are used in thecarbothermal reactor. Both the carbothermal process and the high purity,silicon-containing products made by the process are inventions over theconventional methods and products.

Advantages achievable through use of the inventions disclosed herein caninclude, but are not limited to, energy savings attributable to reducedprocessing times and temperatures, energy production from the recoveryof volatilized carbon, higher processing throughputs, highersilicon-containing product yields, higher removal of impurities, andhigher performance silicon-containing products.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described and explained in relation to FIGS. 1Aand 1B, which together comprise a process flow diagram illustrating insimplified form the various steps of preferred methods by which theproducts disclosed herein are made from a preferred starting materialsuch as rice hulls.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Because of their uniquely high concentrations of silica, rice hulls andrice straw are especially well suited for use in the present invention,and the following description will focus on the use of rice hulls. It isbelieved, however, that other naturally occurring or processed plantmatter containing at least about 1 weight percent silica, and preferablyat least about 3-5 weight percent silica can be similarly used. Ricehulls contain both fixed and volatile carbon, with volatile carbon beingpresent primarily in the form of cellulose. Untreated rice hullstypically have a mole ratio of fixed carbon to silica of about 4:1, avolatile carbon to fixed carbon weight ratio of about 5.24, and amineral content ranging from about one to about three weight percent(10,000 to 30,000 ppm).

The methods of the invention are further described and explained withreference to FIGS. 1A and 1B. Rice hulls are desirably milled andscreened to adjust the average particle size and to open up the hulls sothat they can mix readily with aqueous sulfuric acid solutions asdisclosed herein. The rice hulls are preferably milled to particle sizesranging from about 10 μm to about 1 mm. An average particle size ofabout 150 μm provides good results. If the rice hulls contain dirt orother debris, they are preferably washed and dried before milling. Ricehulls obtained directly from a rice mill are usually relatively cleanand require no washing and drying prior to milling.

Following milling, the rice hulls are preferably leached using anaqueous sulfuric acid solution. Sulfuric acid is believed to beparticularly effective at removing minerals such as phosphorous, foradjusting the ratio of fixed carbon to silica in the leached rice hulls,for reducing the content of volatile carbon, and for increasing theporosity of the leached hulls. Suitable aqueous sulfuric acid solutionscan comprise acid concentrations ranging from about 0.01 weight percentto more than 30 weight percent acid. From about 0.3 to about 1 weightpercent acid is suitable for adjusting the mole ratio of fixed carbon tosilica and for reducing minerals other than iron to very low levels.From about 1 to about 5 weight percent acid is suitable foraccomplishing that which can be done in the low range plus reducing ironto the lowest levels. From about 5 to about 20 weight percent acidconcentration is useful for removal of volatile carbon through thelikely solubilization of cellulose. Sulfuric acid solutions moreconcentrated than about 30 weight percent are more expensive, morecorrosive, and at high concentrations, may produce undesirablebyproducts. By contrast, dilution of the sulfuric acid leaching solutionto concentrations approaching the lower end of the range can reduce theeffectiveness of the acid and increase the time required to achieve thedesired product characteristics.

The solids concentrations that can be satisfactorily used in theleaching method of the invention can range from less than one percent togreater than 35 percent by weight of the resultant mixture, withconcentrations preferably ranging from about one to about 30 weightpercent. A solids concentration in the range of from about 1 to about 10weight percent provides for optimum removal of phosphorous with a singlestage of leaching. However, if extremely low levels of phosphorous arenot critical, then a higher solids range of from about 10 to about 30weight percent provides for reduced water and acid consumption andminimum energy use in drying the carbon-silica product.

The leaching process can be performed in single or multiple stages in abatch or continuous mode, and with various types of reactors and reactorconfigurations as desired. Temperatures below boiling (94-100° C.) aresuitable for removing non-silica minerals, except phosphorous and iron,to low levels, and for adjusting the fixed carbon-to-silica mole ratioto values less than 3:1. Temperatures at boiling (94-100° C.) aid inagitating the mixture of rice hulls and aqueous sulfuric acid.Temperatures ranging from about 100° C. to about 140° C. are needed toreduce phosphorous and iron to their lowest levels and to achieve thelowest fixed carbon-to-silica mole ratios. Temperatures above 140° C.,especially above 160° C. are effective for removing volatile carbon and,at longer reaction times, for increasing fixed carbon so that very highmole ratios of fixed carbon-to-silica ratios can be achieved. The solidsand liquid are preferably mixed continuously during the leaching processso that all the solid particles are contacted by acid solution and sothat heat is distributed throughout the mixture.

Depending upon acid strength, solids concentration, mixing, temperatureand the desired product characteristics, leaching times can range fromas low as about six seconds to about 48 hours or more. Thereafter, theremaining solids are preferably separated from the acid solution andwashed with water. Use of this method increases the measured void volumeof milled rice hulls from low values up to values ranging from at leastabout 30% to about 90%, with values greater than about 50% beinggenerally preferred. At the same time, the leaching method disclosedherein removes a large portion of the minerals that may have originallybeen present in the silica-containing plant matter.

Using the leaching method of the invention, the fixed carbon to silicaratio can be adjusted as desired by controlling the sulfuric acidconcentration, solids concentration, temperature, residence time anddegree of mixing to produce carbon-silica products of high purity, highporosity, high reactivity, fine particle size, and intimately mixedfixed carbon and silica at mole ratios ranging from about 1:1 or less upto about 5:1 or greater. A particular advantage of the subject method isthat products can be selectively produced with fixed carbon-to-silicamole ratios of, for example, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1,4.5:1, 5:1, or any preferred stoichiometric ratio for making varioussilicon-containing products.

The leaching method of the invention can be demonstrated with a benchscale apparatus in which reactions at or below boiling are carried outin a 3-liter Pyrex resin kettle using a temperature controlled heatingmantle. A water-cooled condenser is attached to the top of the kettle toinsure that the acid or solids concentration do not change during thereaction. Reactions above the boiling point of water are carried out ina 2-liter zirconium vessel. In both cases, a thermocouple is used tomeasure reactor temperature and a Teflon device is used to stir thecontents of the reaction vessel. The reaction vessel and othercomponents are preferably cleaned by treating with 50% HCl and thenwashed with demineralized water to remove any trace metal contaminants.

For leaching processes conducted at temperatures at or below boiling,the resin kettle is filled with an aqueous sulfuric acid solution andheated to the desired temperature. Once the kettle is at temperature,milled hulls are added to the kettle to give the desired solidsconcentration. The milled hulls immediately form a uniform mixture inthe kettle and the mixture is periodically sampled. All samples arefiltered and the filtered solids are washed several times withdemineralized water. The washed solids samples are dried and are savedalong with the filtrate for analysis.

For laboratory reactions above boiling, an aqueous sulfuric acidsolution and milled rice hulls are preferably added to a zirconiumvessel. The vessel is sealed and heated with gas-fired heaters to thedesired reaction temperature. Once at reaction temperature, thetemperature is maintained for the duration of the reaction time and thereaction mixture is periodically sampled. At the end of the reactiontime, the vessel is quickly cooled with a water spray to a temperaturebelow boiling. Samples are immediately filtered and then washed severaltimes with demineralized water. The washed solids samples are dried andsaved together with the filtrate for analysis. It will be appreciated bythose of ordinary skill in the art upon reading this disclosure thatvarious types of conventional, commercially available industrialequipment can be used for scaling up the bench-scale experimentalapparatus as described above, but altered as needed to achieve theseresults economically. Also, the order of adding solutions and materialsto the reactor as well as the order of other steps, such as heating thereactor, can be varied to accommodate the scale of operation and thetype of reactor.

Benefits from the leaching step can be accomplished using varying solidsconcentrations with sulfuric acid solutions of various strengths andreacted for varying times and temperatures. The mixing rate duringleaching is desirably such that the solids and liquid are well mixed, sothat acid and heat can be distributed throughout the mixture. Althoughwater alone will produce some beneficial leaching effect, the use ofsulfuric acid is more effective at significantly reducing the mineralcontent, adjusting the fixed carbon to silica mole ratio, reducing thecontent of volatile carbon and improving the porosity of the rice hullswhile retaining the desirable natural features of the rice hulls, whichinclude intimate contact between the carbon and silica, amorphicity ofthe silica and carbon, and very high surface area of the silica andcarbon.

Test results show that the temperature at which the leaching step iscarried out has a significant effect on the process. When leaching withsulfuric acid at temperatures below boiling, sodium, potassium, calcium,magnesium and manganese are effectively leached from rice hull to levelsat or below 10 ppm for each mineral. Although the phosphorous level issignificantly reduced at temperatures below boiling, it is necessary toleach at temperatures above boiling, typically in the range of 120-140°C., to achieve removal of phosphorous to very low levels, such as lessthan 4 ppm. The levels of iron are significantly reduced at temperaturesbelow boiling, but it is necessary to go to temperatures above boiling,typically in the range of about 120° C. to about 160° C. to achieve ironlevels of 10-20 ppm. Because boron is naturally low in rice hulls, theresulting boron concentration is less than 1 ppm. The resulting lowphosphorous and boron contents make this process and the uniquecarbon-silica product very attractive for the production of photovoltaicgrade silicon for use in the production of solar cells, as well as forother industrial applications.

Test results show that temperature also has a significant effect on theremoval of fixed carbon and consequently on the fixed carbon to silicamole ratio. Using sulfuric acid at temperatures as low at 25° C., acertain amount of fixed carbon is solubilized, but to remove enoughfixed carbon to achieve the lower ratios of fixed carbon to silica, itis necessary to leach at temperatures at about boiling or above,preferably around 120° C. Temperature and time also have a significanteffect on the removal of volatile carbon. At temperatures above 160° C.and with relatively short reaction times, a large fraction of thevolatile carbon is solubilized. On the other hand at temperatures above160° C. and longer reaction times the solubilized volatile carbon isconverted to fixed carbon, which is incorporated into the solid,increasing the fixed carbon to silica mole ratio. Although the rate andextent of solubilization of rice hull silica is significantly increasedpercentage-wise with temperature, the overall percent of silica removedfrom the hulls is small at any temperature.

Test results show that the total amount of solids in the mixture duringthe leaching step can also have an effect on the resulting fixed carbonto silica mole ratio. For a given temperature, acid concentration, andtime, solids concentrations approaching 30% will provide higher fixedcarbon to silica ratios than lower solids concentrations forsingle-stage leaching. Higher solids concentrations can also result in ahigher phosphorous content in solids produced by a single-stage leachingoperation. However, higher solids concentrations do not appear to affectthe concentrations of other minerals or have a significant effect on theamount of silica in the solids.

Test results show that differences in practical leaching times do nothave a significant effect on the extent of removal of any of theminerals from rice hulls with sulfuric acid solution. The differences inmineral content between samples collected at very short times of aminute or less and longer times are indistinguishable. However, time andtemperature do affect the removal of fixed carbon. For example, at 94°C., 1% sulfuric acid and 10% solids, it takes about 300 minutes toremove the maximum amount of fixed carbon. By comparison, at 120° C.,the time needed to achieve maximum removal of fixed carbon decreases to120 minutes. Time is also important in the removal of volatile carbonand the conversion of volatile carbon to fixed carbon. For example at200° C., 1% acid, 10% solids a significant amount of volatile carbon isreleased almost instantly providing a volatile carbon to fixed carbonratio of 1.82 compared to 5.24 for rice hulls. As time progresses, thesolubilized volatile carbon is converted to fixed carbon attached to thesolid material. The result is that the volatile carbon to fixed carbonratio continues to decline, reaching values of about 1.12. At the sametime, the fixed carbon-to-silica mole ratio increases to 5:1. Bycomparison, the rate of solubilization of rice hull silica is slower.For example, at 94° C., 1% acid and 10% solids concentrations, it takesover 1400 minutes for the silica solid to achieve equilibrium withsolubilized silica.

Test results show that sulfuric acid preferentially reduces thenon-silica mineral content. Excluding iron, a minimal amount of sulfuricacid is needed to effect the removal of non-silica minerals. Forexample, for a given set of conditions, the maximum possible removal ofnon-silica minerals other than iron is achieved at acid concentrationsas low as 0.3% and many of the minerals are effectively removed with aslittle as 0.06% acid. Higher concentrations of acid are needed to effectmaximum removal of iron. For example, at 94° C. and 10% solids, maximumremoval of iron is achieved at about 2.5% acid.

Acid concentration affects the rate of solubilization of carbon. Forexample, at 94° C., 10% solids, 100 minutes and 1% acid, theconcentration of solubilized carbon in the acid solution is 5-6 gm/L.Under the same conditions at 2.5% acid, the concentration of solubilizedcarbon is 8-9 gm/L. However, the extent of carbon solubilization isessentially the same at both acid concentrations, namely about 12.5 g/L.The effect of acid concentration on the ultimate mole ratio of fixedcarbon-to-silica is relatively minor. However, higher acid concentrationdoes affect the solubilization of volatile carbon as revealed by theeffect of high acid concentrations on the fixed carbon to volatilecarbon weight ratio. For example, at acid concentrations between 0.06%and 10% the ratio is between 0.16:1 and 0.2:1, whereas at 20% acidconcentration the ratio is 0.3, indicating removal of volatile carbon isenhanced somewhat at the higher acid concentration. Sulfuric acidconcentration also affects the rate, as opposed to the extent, of silicasolubilization. For example, at 94 deg C., 10% solids and 0.30% acid,the concentration of silica in the acid solution is 0.12 gm/L at 180minutes. At 1% acid, the concentration of silica in the acid solution is0.28 gm/L at 180 minutes. The ultimate concentration of rice hull silicaat 94 deg C., regardless of the acid concentration, appears to about0.450 gm/L.

Test data shows that aqueous sulfuric acid solution, under the rightconditions of acid concentration, temperature, time and solidsconcentration, can dissolve a very significant amount of the minerals inrice hulls, can remove significant portions of the fixed carbon andvolatile carbon, can provide for converting the solubilized volatilecarbon to fixed carbon, and can increase the porosity of rice hulls. Assilica is not very soluble in the aqueous sulfuric acid, this has theeffect of reducing or increasing the mole ratio of fixed carbon tosilica in the composition, removing minerals, removing volatile carbonand increasing the porosity of the material. Based upon this disclosure,one of ordinary skill in the art can, without undue experimentation,adjust these reaction parameters to provide the desired mole ratio offixed carbon to silica, the desired mineral purity, the desired volatilecarbon content and the desired porosity. For example, a fixed carbon tosilica mole ratio of about 2:1 and a very low level of minerals (lessthan 100 ppm total, less than 4 ppm phosphorous, and 1 ppm or lessboron) can be achieved with temperatures between about 94° C. and about140° C., sulfuric acid concentrations of about 1% to about 5% and solidsconcentrations of about 10% in about two hours. A fixed carbon to silicamole ratio of about 3:1 and a very low level of minerals can be achievedat temperatures in the range of about 150° C., sulfuric acidconcentrations of about 1% to about 5% and solids concentrations ofabout 10% in about 4 hours. In addition, the leaching conditionspreserve the intimate relationship between the silica and the fixedcarbon, the amorphicity of the carbon and silica, the high surface areaof the silica and carbon and provide for an overall increase inmeasurable void volume. If a fixed carbon to silica mole ratio of 3:1 isdesired, but the levels of phosphorous and iron are not critical, atemperature of less than 94° C. can be used with acid and solidsconcentration described above to achieve the sought ratio.

The high purity (removal of over 99.7% of the minerals) carbon-silicaproduct produced by the leaching process contains a mixture of fixedcarbon and silica, together with varying amounts of volatile carbon. Thecarbon and silica are intimately mixed at least on a micron, andgenerally on a nanometer, scale. The carbon-silica product contains highporosity, small average particle size, and is highly reactive. Theincreased porosity in indicated by the fact that the accessible voidvolume of the milled rice hull particles is very low, but, when thematerial is treated with 1% sulfuric acid at 94° C., the accessible voidvolume increases to 80%. The high reactivity of the produced material isestablished by the fact that the material undergoes carbothermalreduction significantly faster than when conventional feedstocks areused. Conventional feedstocks generally take a day or more to undergocarbothermal reduction. In contrast, the carbon-silica productsdisclosed herein undergo carbothermal reduction for production ofsilicon carbide in under 75 minutes and within 5-7 hours for siliconnitride. This results in a substantial energy savings and greaterthroughput for operation of the carbothermal reactor.

The leaching method of the invention can also be practiced in multiplestages. The first leaching stage can be used to remove a large portionof the minerals and make adjustments to the fixed carbon to silica moleratio. The washed or unwashed material is then contacted a second timein a sulfuric acid solution. Thereafter, the remaining solids areseparated from the acid solution by filtration or another similarlyeffective means, and washed with water. The second leaching stage can beused to reduce the mineral impurities to a lower level than was achievedwith a single-stage leaching to further adjust the mole ratio of fixedcarbon to silica to a desired ratio ranging between about 5:1 to lessthan 2:1, and to reduce the level of volatile carbon. Additional stagesof leaching can likewise be added to reduce the mineral impurity levelof the solids even more while continuing to make adjustments in thefixed carbon to silica mole ratio and volatile carbon content.

As an example, which should not be construed as the only application ofa multiple-stage leaching method, two-stage leaching is useful when itis desirable to carry out the leaching process at high solidsconcentrations while achieving a low fixed carbon to silica ratio andvery low non-silica mineral content, including very low levels ofphosphorous. As noted above, for a given temperature, acid concentrationand time, higher solids concentrations (approaching 30%) provide higherfixed carbon-to-silica ratios than lower solids concentrations forsingle-stage leaching. Higher solids concentrations also result in ahigher phosphorous content than lower solids concentrations forsingle-stage leaching. However, if two-stage leaching is used at highsolids concentrations, the first stage will reduce the fixedcarbon-to-silica ratio to 3:1 or less (but greater than 2:1) and willreduce all the non-silica minerals except phosphorous to very lowlevels. The phosphorous will be significantly reduced compared to thestarting values, but will remain higher than desired. If, for example,the first stage is operated at 25% solids, 1% acid, and 140° C. for 15minutes, the fixed carbon-to-silica ratio is reduced to between 3:1 and2.5:1. The levels of sodium, potassium, magnesium, and manganese arereduced to less than 10 ppm. The level of calcium is 10 ppm and thelevel of iron is 20 ppm. However phosphorous is at 100 ppm. With asecond stage of leaching at the same solids concentration, temperatureand acid concentration as the first stage, the fixed carbon-to-silicamole ratio is further reduced to low values of around 2:1 or less, andphosphorous is reduced to very low values of less than 4 ppm.

In another application of the leaching process for the sole purpose ofreducing the phosphorous content to a low value, for example,phosphorous levels are reduced by more than 99.96%, to less than 4 ppmwith a single stage of leaching at 120° C., 10% solids, 1% acid and atime of 15 minutes or less.

In another application of the leaching process for the sole purpose ofreducing the iron content to a low value, for example, the iron levelsare reduced by over 92% to values less than 20 ppm with a single stageof leaching at 140° C., 28.6% solids, 1% acid and a time of 15 minutesor less.

In another application of the leaching process for the sole purpose ofreducing total minerals to overall low values, for example, the totallevel of sodium, potassium, magnesium, calcium, manganese, iron andphosphorous is reduced to less than 79 ppm with a single stage ofleaching at 120° C., 10% solids, 1% acid and a time of 15 minutes orless.

In another application of the leaching process for the sole purpose ofremoving fixed carbon, for example, the fixed carbon to silica moleratio of the rice hulls is reduced from 3.88:1 to 3:1 using a singlestage of leaching at 25° C., 10% solids, 1% acid and times ranging from1 minute to 60 minutes. On the other hand, this ratio can be reduced to2:1 with a single stage of leaching at 94° C., 10% solids, 1% acid and atime of 300 minutes.

In another application of the leaching process, using either singlestage or multiple stages for the sole purpose of reducing the volatilecarbon content, for example, at 200° C. at very short reaction times,where significant fixed carbon has been removed, the volatile carbon tofixed carbon mole ratio drops to 1.82 compared to values of 5.24 in rawrice hulls in spite of the reduction of the amount of fixed carbon. Itis likely that, at this temperature and short reaction time, asignificant amount of the cellulose, which is a form of volatile carbon,has been solubilized.

In another application of the leaching process, either single stage ormultiple stage leaching is used for the sole purpose of increasing thefixed carbon. For example, the native fixed carbon to silica mole ratioof rice hulls used in this study is 3.88:1. This ratio can be increasedto 5:1 with a single stage of leaching at 200° C., 10% solids and 1%acid in 120 minutes. As another example, if material is prepared from asingle stage of leaching that has a fixed carbon to silica ratio of 2:1,the mole ratio of fixed carbon to silica can be increased to any highervalue up to about 5:1 by operating a second stage of leaching at atemperature above 160° C. for longer reaction times.

Another application of this invention relates to use of the leachingprocess, either single stage or multiple stages, for the sole purpose ofincreasing the porosity. For example the accessible void volume inmilled hulls is rapidly increased to 30% after 15 minutes of leaching at94° C. and 1% acid. The void volume increases yet further to 80% after30 minutes of leaching under the same conditions.

Another application of the invention relates to use of the leachingprocess, either single stage or multiple stages, for a combination ofany two or more of: removing phosphorous, iron or other minerals fromnaturally occurring or processed plant matter containing more than 3weight percent silica; adjusting the mole ratio of fixed carbon tosilica; removing volatile carbon; converting volatile carbon to fixedcarbon; and increasing the porosity of the resultant carbon-silicaproducts.

Another application of this invention relates to a preferred method forthermally volatilizing the volatile carbon component of thecarbon-silica product in a reactor separate from the carbothermalreactor. This reaction can be performed in a bench-scale alumina reactorfitted with a metal head plate, and can be scaled up for industrialapplication using conventional process equipment. If desired, othermaterials such as graphite can be used to construct the reactor. Thebench-scale reactor is three feet long and six inches in diameter. Thecarbon-silica product is contained in an alumina or graphite cruciblethat was located at the bottom of the reactor. The lower portion of thereactor is positioned in an electric furnace that is controlled by aprogram that allows different rates of temperature rise. The furnaceheats the carbon-silica product from ambient temperature to over 1,000°C. Argon or nitrogen enters the top of the reactor and is directed to aparticular level in the reactor through an adjustable tube. Volatilizedcarbonaceous material exits the reactor through the top. A slipstream ofthis material is sent to a CO monitor, and the majority of the materialis directed to a condenser where the condensables are separated from thegases. Volatilization starts at about 400° C. and reaches a peak atabout 600° C. Subsequently, the carbon monoxide levels decrease,reaching a base line at about 950° C. This profile differs from thosereported for raw rice hulls in that, with raw rice hulls, thevolatilization process starts at about 250° C. It is clear that thematerial that normally volatilizes at lower temperatures has beenremoved by the leaching process from the carbon-silica product. Thevolatilized gases and vapors can be captured and used as an energysource for the leaching process, the carbothermal process or otherprocesses. A further benefit is that a portion of the remaining mineralcontent is also volatilized with the volatile carbon. The volatilizedmineral can be collected in order to further purify the resultingdevolatilized carbon-silica product. The resulting material is in theform of thermal-devolatilized carbon-silica products, which are uniqueproducts and are themselves an embodiment of this invention.

Alternatively, according to another preferred embodiment, most if notall of the volatile carbon is removed by further treating the uniquecarbon-silica products produced from the leaching step with dilutesulfuric acid at temperatures ranging from about 160° C. up to about200° C. The residence time at the higher temperatures is very short, onthe order of about 1 minute. This significantly lowers the volatilecarbon content with minimal effect on the fixed carbon content of thesolids. This method also removes a portion of the residual minerals leftin the carbon silica products derived from the leaching step, enhancingthe purity of the resulting high purity acid devolatilized carbon-silicaproducts. The remaining solids are separated from the acid solution andthe solids are washed with water. The resulting solid material is a highpurity, devolatilized carbon-silica product with a predetermined fixedcarbon-to-silica ratio and mineral impurities well below 0.1 weightpercent.

A variety of unique acid devolatilized carbon-silica products, each ofwhich has a predetermined molar ratio of fixed carbon to silica,contains very low levels of minerals and are highly porous and highlyreactive, can be produced by the leaching step followed by an aciddevolatilization step. The products preferably have a desired mole ratioof fixed carbon to silica from around 3:1 to less than 2:1, have a totalmineral content less than 100 ppm, have a boron content less than 1 ppm,and have a phosphorous content less than 4 ppm. The products alsopreferably are highly porous and highly reactive and contain silica andfixed carbon that are intimately mixed at the micron and/or nanometriclevel. These unique materials can be produced in a fine powder form ormade into pellets without the need for external binders. The discloseddevolatilized carbon-silica products are preferred for use in thesubsequent production of silicon containing products, including, but notlimited to, silicon carbide, silicon nitride, silicon, and silicontetrachloride.

Following leaching, washing and drying, the carbon-silica products ofthe invention are frangible and easily reduced in size with minimalenergy consumption to particles sizes in the fine (500 μm to 50 μm),very fine (50 μm to 5 μm) and ultrafine (5 μm to submicron) categories.The average size and size distribution of the resulting particles is setby the physical characteristics desired in the final silicon-containingproducts. Milling devices suitable for this purpose include, forexample, batch impeller-type stirred ball mills, continuous disc-typestirred ball mills, continuous vibrating ball mills, batch verticalvibrating ball mills, and fluid energy mills such as a continuous loopmill or a continuous pancake mill.

Either a carbon-silica product or a devolatilized carbon-silica productcan be used as a powder or, in another embodiment of this invention, canbe pelletized for use in subsequent processes. The conventional methodof producing pellets involves mixing silica and carbon compounds with abinding agent, such as sugar, and then compressing the mixture into apellet. The use of a conventional binding agent increases the cost ofprocessing and the binding agent may introduce impurities into thepelletized product. In contrast, the unique carbon-silica products andthe devolatilized carbon-silica products of the present invention can bepelletized without the use of an additional binder. Without limiting theinvention, it is believed that the lignin and cellulose that are presentin the product can act as a sufficient binding agent for the silica andcarbon. In contrast to conventional pellets that are generally onlyadmixed on a macro scale, use of the current invention results inpellets that have carbon and silica intimately admixed at the μm andnanometer scale. This intimate mixture of highly reactive forms of thecarbon and silica facilitate production of a pellet that reacts morequickly and completely during the subsequent carbothermal reaction thanconventional pellets. The pelletized products made from carbon-silicaproducts and/or devolatilized carbon-silica products as disclosed hereincontain a desired mole ratio of fixed carbon to silica and/or very lowlevels of minerals, especially low levels of phosphorous and iron, andconstitute another embodiment of the invention.

Referring again to FIGS. 1A and 1B, the carbothermal reduction method ofthe invention generally involves heating the carbon-silica productand/or the devolatilized carbon-silica product in a carbothermal reactorto a very high temperature to initiate the reduction of the silica tothe silicon-containing product. Carbothermal reactors generally willheat the carbon-silica composition to about 1350-1800° C., dependingupon the silicon-containing product of interest. For silicon or siliconcarbide, an inert atmosphere is used. For other products, differentatmospheres are used in the carbothermal reactor. For example, nitrogenor nitrogen plus hydrogen or nitrogen plus ammonia can be used toproduce silicon nitride, whereas chlorine can be used to produce silicontetrachloride. The reaction times are extremely fast compared toconventional carbothermal processes that use conventional sources ofsilica and carbon. Using carbon-silica products and/or devolatilizedcarbon-silica products, reaction times are on the order of minutes to afew hours, depending upon the desired silicon-containing product. Thiscompares to times of a day or more for most conventional carbothermalprocesses that use conventional sources of silica and carbon. The fastreaction time provides for significantly lower use of energy and higherthroughput compared to conventional carbothermal processes that useconventional sources of silica and carbon. In addition, because thecarbon silica product and devolatilized carbon silica product areavailable in the form of fine powder, these products can be readily usedin advanced carbothermal reactor systems, whereas conventional sourcesof carbon and silica cannot. Examples of commercial equipment that canbe used to implement the advanced carbothermal processes disclosedherein include, but are not limited to, static bed reactors, moving bedreactors, fluidized bed reactors; rotary tube reactors, laser reactors,flame reactors, furnace reactors employing an external heat source; andplasma reactors. Many of these advanced reactors can provide forisothermal conditions and controlled reaction times, both of which areimportant for silicon-containing product quality.

Additionally, because the novel carbon-silica products of the inventioncan, if desired, contain significant amounts of volatile carbon, use ofthese unique materials affords the opportunity to recover significantvolatile carbon during the carbothermal process and use such materialfor energy purposes or other valuable purposes. The use of energy fromthe volatilized carbon can reduce and perhaps eliminate the need toacquire energy from an outside source to operate the carbothermalprocess. A further benefit is that a portion of the remaining mineralcontent is also volatilized along with the volatile carbon. Thevolatilized minerals can be separately collected in order to furtherpurify the final silicon-containing product.

According to a preferred carbothermal method of the invention usingbench-scale apparatus, the reaction is performed in an alumina reactorfitted with a metal head plate. Other materials, including graphite, canbe used to construct the reactor. The reactor is three feet long and sixinches in diameter. The carbon-silica product or the devolatilizedcarbon-silica product is contained in an alumina or graphite cruciblethat is located at the bottom of the reactor. The lower portion of thereactor is positioned in an electric furnace that is controlled by aprogram that allows different rates of temperature rise. The furnacegenerally heats the carbon-silica product to a temperature ranging fromabout 1250 to about 2200° C., preferably from about 1350 to about 1800°C. depending upon the type of silicon-containing product to be produced.Inert, and/or reactive gas enters the top of the reactor and is directedto a particular level in the reactor through an adjustable tube. If thefeed material used is carbon-silica product containing a certain amountof volatile carbon, as opposed to a devolatilized-carbon-silica product,volatilized carbonaceous material exits the reactor through a gas outlettube attached to the metal head plate. A slipstream of the off-gas issent to a CO monitor, and the majority of the material is directed to acondenser where the condensables are separated from the gases.

The atmosphere used in the carbothermal reactor depends upon thesilicon-containing product that is desired. For silicon carbide andsilicon, argon can be used. To produce silicon nitride, pure nitrogen or90% nitrogen plus 10% hydrogen or 90% nitrogen plus 10% ammonia can beused. Likewise, a chlorine atmosphere can be used to produce silicontetrachloride.

Due to the loss of SiO gas from the carbothermal reactor and/or thedesire to have carbon in the silicon-containing product, and/or thedesire to have excess carbon fed to the reactor to insure completeconversion of the non-carbon reactants, the actual desired mole ratio offixed carbon to silica will generally be different from the theoreticalvalue. For example, the theoretical fixed carbon to silica mole ratiofor production of silicon carbide is 3:1 and the theoretical ratio forproduction of silicon and silicon nitride is 2:1. However, the optimumratio for a particular reactor may be less than the theoretical value,depending on the efficiency of the particular reactor in terms of lossof SiO gas. This invention provides the means to produce material with acarbon to silica ratio that is “rich” in silica compared to thetheoretical ratio to accommodate the loss of SiO. Alternatively, it maybe desirable to have a certain percentage of carbon left in thesilicon-containing product and this invention provides the means toproduce material with a carbon-to-silica ratio that is “rich” in carboncompared to the theoretical ratio in order to accommodate the desire forleftover carbon. Moreover, sometimes it is desirable to have “extra”carbon to insure that the other reactants are fully reacted and, asalready noted, this invention provides the means to produce a carbonrich carbon-silica product and/or carbon rich devolatilized carbonsilica product for such applications. If two or three of the abovesituations exist or are desired, this invention provides the means toproduce a carbon-silica product or devolatilized carbon-silica productwith the optimum carbon-to-silica mole ratio for the given situation.

The process and products of the current invention are furtherillustrated by the examples below. Milled rice hulls and the solid andliquid samples were analyzed by a variety of methods. Proximate analysiswas used to determine percent moisture, percent volatile carbon, percentfixed carbon, and percent ash in the solids. The solids were alsoanalyzed for total carbon and for SiO₂. The solid samples were alsoanalyzed for mineral content using spark source mass spectroscopy, glowdischarge mass spectroscopy, atomic absorption spectroscopy, and x-rayfluorescence. The Malvern method was used to determine particle sizedistribution. Specific surface area was calculated using the BET methodand porosity using the mercury porosimetry method. A coulometric methodwas used to analyze the liquid filtrate samples for total organic carbon(TOC), a blue colorimetric method was used to analyze for phosphorous,and atomic absorption was used to analyze for minerals.

Example 1

Rice hulls were obtained from a mill in Arkansas. The hulls were mixedand then milled in a ring and puck mill to achieve an average particlesize of 138 μm. The milled hulls were analyzed via proximate analysisand for SiO₂ content. They were found to contain 7.17% moisture, 62.72%volatile carbon, 11.97% fixed carbon and 18.14% ash. The ash was 85%silica (SiO2). The fixed carbon to silica ratio (mole:mole) was 3.88.The volatile carbon to fixed carbon ratio (wt %:wt %) was 5.24. Themilled hulls were also analyzed for mineral content by spark source massspectroscopy and contained amounts of the specified minerals as shown inTable 1.

TABLE 1 Element ppm Na 400 K >10,000 Mg >2000 Ca 2000 Mn >2000 Fe 250 B1 P >10,000 Total >26,651A 1% sulfuric acid solution was prepared using demineralized water. Theacid solution containing 10% milled rice hulls was added to a 2 literzirconium reactor and heated to 120° C. Once at reaction temperature,the contents of the reactor were sampled frequently. The samples wereimmediately filtered and washed with demineralized water. The solidswere dried and saved for analysis along with the filtrate.

The dried solids were analyzed by proximate analysis and silicaanalysis. The analyses showed that the fixed carbon to silica mole ratioat 120 minutes was 2.04:1, which is essentially the theoretical ratiofor production of silicon and silicon nitride. The solids, analyzed byatomic absorption spectroscopy and calorimetric tests, contained thefollowing non-silica mineral composition, shown in Table 2:

TABLE 2 Element Ppm Na <10 K <10 Mg <10 Ca 10 Mn 10 Fe 25 P <4 Total <79Thus, the overall percent removal of the specified minerals using asingle stage of leaching was greater than 99.7%, giving a product with aspecified mineral impurity less than 79 ppm. In addition, it was foundthat sulfuric acid, at the temperature and acid concentration used inthe test with a single stage of leaching, is very effective at removingphosphorous to very low levels of less than 4 ppm. The resultingcarbon-silica material was in the form of a fine powder with a voidvolume of about 80%. In other carbon-silica products leached in asimilar manner, the boron level is 1 ppm as measured by spark sourcemass spectroscopy.

Example 2

A 1% sulfuric acid solution was prepared using demineralized water. Theacid solution containing 10% milled rice hulls was added to a 2 literzirconium reactor and heated to 160° C. Once at reaction temperature,the contents of the reactor were sampled frequently. The samples wereimmediately filtered and washed with demineralized water. The solidswere dried and saved for analysis along with the filtrate.

The dried solids were analyzed by proximate analysis and silicaanalysis. The analyses showed that the fixed carbon to silica mole ratioat 180 minutes was 3:1, which is the theoretical ratio for theproduction of silicon carbide. The solids, analyzed by atomic absorptionspectroscopy and colorimetric tests, contained amounts of the specifiedminerals as shown in Table 3:

TABLE 3 Element Ppm Na <10 K <10 Mg <10 Ca <10 Mn <10 Fe 20 P <4 Total<79Thus, the overall percent removal of the specified minerals using asingle stage of leaching was greater than 99.7%, yielding a product witha mineral impurity level less than 79 ppm for the specified minerals. Inaddition, it was found that sulfuric acid, at the temperature and acidconcentration used in the test with a single stage of leaching, is veryeffective at removing phosphorous to very low levels less than 4 ppm. Inother carbon-silica products leached in a similar manner the boron levelis 1 ppm as measured by spark source mass spectroscopy. The resultingcarbon-silica material was in the form of a fine powder with a voidvolume of about 80%.

Example 3

A 1% sulfuric acid solution was prepared using demineralized water. Theacid solution was added to a 3-liter Pyrex resin kettle and heated to94° C. Once at temperature, milled rice hulls were added to the kettleto create a mixture with 10% solids. The milled hulls were immediatelyincorporated into the acid solution producing a well-mixed slurry. Thereaction was carried out for five minutes. At the end of this time, thecontents of the kettle were filtered and washed with demineralizedwater. The resulting solid material was dried. This procedure wasrepeated four more times. The dried solids from each run were combined.

The fixed carbon-to-silica mole ratio of the combined carbon-silicaproduct was 2:51:1, which is between the theoretical ratio for producingsilicon carbide and the theoretical ratio for producing silicon nitride.Malvern analysis of the material indicated a d50 particle diameter of114 um.

Example 4

The material prepared in Example 3 was used to test the production ofsilicon carbide. According to a preferred carbothermal method of theinvention using bench-scale apparatus, the reaction was performed in thealumina reactor described previously. The reactor was purged with pureargon. After the air was purged from the reactor, the temperature wasincreased from ambient to the final carbothermal reaction temperature of1,550° C. The purge with argon was continued. The carbon monoxide in theoff-gas was monitored and it was observed that volatilization ofvolatile carbon started at about 400° C. A carbon monoxide peak wasreached at about 600° C. and the carbon monoxide level declined,reaching a base line at about 950° C. At about 1,300° C., the carbonmonoxide level began to increase, indicating the start of the reactionbetween fixed carbon and silica, and reached a peak at 1,550° C. Soonafter reaching 1,550° C., the carbon monoxide level began to decreaserapidly, reaching a baseline level in about 60 minutes. The totalelapsed time between when the production of silicon carbide started andwhen it ended was about 75 minutes, which was much shorter than thereaction time for the commercial Acheson process, which reaction time istypically about 36 hours. As determined by x-ray diffraction, theresulting product contained only silicon carbide and no othercrystalline material.

The silicon carbide produced by the present invention has uniquefeatures that are of significant value in making high end ceramics. Forexample, the silicon carbide is composed of microcrystals withdimensions of about 15 nanometers. These microcrystals are grouped intoparticles that are about 200 nanometers in diameter. Silicon carbideparticles in the nanometric range are sought for the production of highend silicon carbide ceramics and other high-end applications. Thisunique carbothermal process involving the unique carbon-silica productas feedstock and the unique silicon carbide product are embodiments ofthis invention

Example 5

The material prepared in Example 3 was used to produce silicon nitride.According to a preferred carbothermal method of the invention usingbench-scale apparatus, the reaction was performed in the alumina reactordescribed previously. The reactor was purged with pure nitrogen and thetemperature was increased from ambient to the final carbothermalreaction temperature of 14250 while continuing the nitrogen purge. Thecarbon monoxide in the off-gas was monitored and it was observed thatvolatilization of volatile carbon started at about 400° C. A carbonmonoxide peak was reached at about 600° C. and the carbon monoxide levelthen declined, reaching a base line at about 950° C. At about 1300° C.,the carbon monoxide level again began to increase, indicating the startof the reaction between fixed carbon and silica, and reached a peak at1425° C. Soon after reaching 1425° C., the carbon monoxide level againbegan to decline, slowly reaching a baseline level. The total elapsedtime between when the production of silicon nitride started and when itended was about 5-7 hours. As determined by x-ray diffraction, theresulting product contained only silicon nitride and no othercrystalline material. The silicon nitride is a mixture of acicularcrystals with a very high aspect ratio and small particles. Siliconnitride consisting of long acicular crystals mixed with small particlesis a self-reinforcing form of silicon nitride that is ideal forproducing superior silicon nitride ceramics and for other high-endapplications. This carbothermal process involving the uniquecarbon-silica product as feedstock and the unique silicon nitrideproduct produced by the process are embodiments of this invention.

Example 6

The material prepared in Example 3 was used to produce silicon nitride.According to a preferred carbothermal method of the invention usingbench-scale apparatus, the reaction was performed in the alumina reactoras described in Example 5 above, except that a mixture of 90% nitrogenand 10% hydrogen was used in place of 100% nitrogen. The total elapsedtime was again about 5-7 hours, which was much shorter than the reactiontime for the commercial carbothermal process, which time is measured indays. As determined by x-ray diffraction, the resulting productcontained only silicon nitride and no other crystalline material. Theyield of silicon nitride was about 2 times that obtained with a purenitrogen atmosphere. The silicon nitride was a mixture of smallparticles only. Silicon nitride consisting of small particles is soughtafter for the production of high-end ceramics and other high-endapplications. This carbothermal process involving the uniquecarbon-silica product as feedstock and the unique silicon nitrideproduct produced by the process are embodiments of this invention.

Example 7

The material prepared in Example 3 was used to produce silicon nitride.According to a preferred carbothermal method of the invention usingbench-scale apparatus, the reaction was performed in the alumina reactoras described in Example 5 above, except that a mixture of 90% nitrogenand 10% ammonia was used in place of 100% nitrogen. The total elapsedtime was again about 5-7 hours, which was much shorter than the reactiontime for the commercial carbothermal process, which time is measured indays. As determined by x-ray diffraction, the resulting productcontained only silicon nitride and no other crystalline material. Theyield of silicon nitride was comparable to that achieved with a purenitrogen atmosphere. The silicon nitride is a mixture of acicularcrystals with a moderate aspect ratio and small particles. This type ofmaterial is a self-reinforcing form of silicon nitride that is ideal forthe production of superior ceramics and other high-end applications.This carbothermal process involving the unique carbon-silica product asfeedstock and the unique silicon nitride product produced by the processare embodiments of this invention.

When carbon-silica products as disclosed herein are used in thecarbothermal process of the invention, remaining volatile carbon isreleased at temperatures below the temperature needed to effect thereaction between fixed carbon and silica to make the desiredsilicon-containing product. This released volatile carbon can becaptured and used for fuel, either to provide energy to the carbothermalprocess or to some other process such as, for example, the leachingprocess. Further devolatilization of the carbon-silica products of theinvention inside the reactor will also further reduce the impuritiespresent in the resultant silicon-containing product because some of thecontaminating minerals and metals will be volatilized and removed alongwith the volatile carbon.

The above-described processes and the unique carbon-silica products anddevolatilized carbon silica products created therefrom can be used inother known reactor configurations or carbothermal processes to produceother types of silicon-containing products. It is expected that theunique characteristics of the carbon-silica products and thedevolatilized carbon-silica products will result in products of asuperior quality and/or significant time, energy, and cost savings inthe creation of a wide variety of silicon containing products, whencompared to conventional carbothermal processes using conventionalmaterial inputs.

In a conventional carbothermal process, the products that are madeearlier in the process have to sit in the reactor at temperature forvery long times. Because of this, the conventional products begin tosinter, forming larger agglomerated particles. In addition, thecrystalline phase changes and, as a result, the product quality iscompromised. With the processes disclosed herein, the faster reactiontimes provide a shorter residence time insider the reactor attemperature and there is much less chance for the undesirable secondaryeffects to occur, meaning that the particle size and crystalline phaseof the products are not compromised. Also, because faster reaction ratestypically produce higher product yields, it is expected that thecarbothermal process practiced as disclosed herein, using the improvedcarbon-silica products or devolatilized carbon-silica products of theinvention as feed material, will produce better product yields comparedto conventional carbothermal processes. Moreover, because thecarbon-silica products and devolatilized carbon-silica products used asstarting materials are very pure, the resulting silicon-containingproducts are also very pure.

The above descriptions of certain embodiments are made for illustrativepurposes only and are not intended to be limiting in any manner. Otheralterations and modifications of the preferred embodiment will becomeapparent to those of ordinary skill in the art upon reading thisdisclosure, and it is intended that the scope of the invention disclosedherein be limited only by the broadest interpretation of the appendedclaims to which the inventors are legally entitled.

1. A method for making carbon-silica products comprising the steps of: providing plant matter comprising at least about 3 weight percent of silica, non-silica minerals and metals; contacting the plant matter with an aqueous solution of sulfuric acid having an acid concentration ranging from about 0.01 weight percent to about 30 weight percent to produce a mixture of plant matter and acid solution; reacting the mixture at a temperature ranging from about 10° C. to about 250° C. for a reaction period ranging from about six seconds to about 48 hours, thereby leaching minerals out of the plant matter to produce a carbon-silica product having an adjusted mole ratio of fixed carbon to silica of at least about 1.0:1; and thereafter removing the acid solution from the carbon-silica product.
 2. The method of claim 1 wherein the plant matter is selected from the group consisting of naturally occurring plant matter, processed plant matter, and mixtures thereof.
 3. The method of claim 2 wherein the plant matter is selected from the group consisting of rice hulls, rice straw and mixtures thereof.
 4. The method of claim 3 wherein the plant matter is rice hulls.
 5. The method of claim 1 wherein the particle size of the plant matter is reduced prior to contacting it with the aqueous acid solution.
 6. The method of claim 5 wherein the plant matter has a particle size ranging from about 10 microns to about 1 mm.
 7. The method of claim 6 wherein the plant matter has an average particle size of about 150 microns after being reduced in size.
 8. The method of claim 1 wherein the mixture of plant matter and aqueous acid solution is mixed.
 9. The method of claim 8 wherein the mixing is continuous.
 10. The method of claim 1 wherein the solution of aqueous sulfuric acid is heated before contacting the plant matter.
 11. The method of claim 1 wherein the mixture of plant matter and aqueous sulfuric acid is heated after contacting the plant matter.
 12. The method of claim 1 wherein the mixture is reacted at a temperature less than 94° C.
 13. The method of claim 1 wherein the mixture is reacted at a temperature of at least 94° C.
 14. The method of claim 1 wherein the mixture is reacted at a temperature ranging from about 94° C. to about 250° C.
 15. The method of claim 14 wherein the mixture is reacted at a temperature ranging from about 94° C. to about 105° C.
 16. The method of claim 14 wherein the mixture is reacted at a temperature ranging from about 105° C. to about 140° C.
 17. The method of claim 14 wherein the mixture is reacted at a temperature ranging from about 140° C. to about 160° C.
 18. The method of claim 14 wherein the mixture is reacted at a temperature ranging from about 160° C. to about 250° C.
 19. The method of claim 1 wherein the plant matter comprises from less than 1 weight percent to more than about 35 weight percent of the mixture.
 20. The method of claim 19 wherein the plant matter comprises from about 1 weight percent to about 30 weight percent of the mixture.
 21. The method of claim 19 wherein the plant matter comprises from about 1 weight percent to about 10 weight percent of the mixture.
 22. The method of claim 19 wherein the plant matter comprises from about 10 weight percent to about 30 weight percent of the mixture.
 23. The method of claim 1 wherein the plant matter comprises about 10 weight percent of the mixture.
 24. The method of claim 1 wherein the plant matter comprises about 25 weight percent of the mixture.
 25. The method of claim 1 wherein the plant matter comprises about 29 weight percent of the mixture.
 26. The method of claim 1 wherein the aqueous acid solution has an acid concentration ranging from about 0.06 weight percent to about 1 weight percent.
 27. The method of claim 1 wherein the aqueous acid solution has an acid concentration ranging from about 1 weight percent to about 5 weight percent.
 28. The method of claim 1 wherein the aqueous acid solution has an acid concentration ranging from about 5 weight percent to about 10 weight percent.
 29. The method of claim 1 wherein the aqueous acid solution has an acid concentration ranging from about 10 weight percent to about 20 weight percent.
 30. The method of claim 1 wherein the aqueous acid solution has an acid concentration ranging from about 20 weight percent to about 30 weight percent.
 31. The method of claim 1 wherein the reaction period ranges from about 6 seconds up to 1 minute.
 32. The method of claim 1 wherein the reaction period is greater than 1 minute and less than about 30 minutes.
 33. The method of claim 1 wherein the reaction period is greater than 30 minutes and ranges up to 12 hours.
 34. The method of claim 1 wherein the reaction period is greater than 12 hours and ranges up to about 48 hours.
 35. The method of claim 1 wherein the carbon-silica product is washed and dried.
 36. The method of claim 1 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio ranging from about 1.0:1 to about 5:1.
 37. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 1.5:1.
 38. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 2:1.
 39. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 2.5:1.
 40. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 3:1.
 41. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 3.5:1.
 42. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 4:1.
 43. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 4.5:1.
 44. The method of claim 36 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio of about 5:1.
 45. The method of claim 1 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio less than about 5:1.
 46. The method of claim 1 wherein the carbon-silica product has a fixed carbon-to-silica mole ratio greater than about 5:1.
 47. The method of claim 1 wherein the carbon-silica product comprises less than about 1000 ppm by weight minerals.
 48. The method of claim 47 wherein the carbon-silica product comprises less than about 500 ppm by weight minerals.
 49. The method of claim 48 wherein the carbon-silica product comprises less than about 200 ppm by weight minerals.
 50. The method of claim 49 wherein the carbon-silica product comprises less than about 100 ppm by weight minerals.
 51. The method of claim 1 wherein the carbon-silica product comprises less than about 10 ppm by weight phosphorous.
 52. The method of claim 51 wherein the carbon-silica product comprises less than about 4 ppm by weight phosphorous.
 53. The method of claim 1 wherein the carbon-silica product comprises less than about 10 ppm by weight sodium.
 54. The method of claim 1 wherein the carbon-silica product comprises less than about 10 ppm by weight potassium.
 55. The method of claim 1 wherein the carbon-silica product comprises less than about 10 ppm by weight magnesium.
 56. The method of claim 1 wherein the carbon-silica product comprises less than about 20 ppm by weight calcium.
 57. The method of claim 56 wherein the carbon-silica product comprises less than about 10 ppm by weight calcium.
 58. The method of claim 1 wherein the carbon-silica product comprises less than about 10 ppm by weight manganese.
 59. The method of claim 1 wherein the carbon-silica product comprises less than about 30 ppm by weight iron.
 60. The method of claim 59 wherein the carbon-silica product comprises less than about 20 ppm by weight iron.
 61. The method of claim 60 wherein the carbon-silica product comprises less than about 10 ppm by weight iron.
 62. The method of claim 1 wherein the carbon-silica product comprises less than about 5 ppm by weight boron.
 63. The method of claim 62 wherein the carbon-silica product comprises less than about 2 ppm by weight boron.
 64. The method of claim 63 wherein the carbon-silica product comprises less than about 1 ppm by weight boron.
 65. The method of claim 1 wherein the carbon-silica product comprises less than about 50 ppm by weight aluminum.
 66. The method of claim 65 wherein the carbon-silica product comprises less than about 20 ppm by weight aluminum.
 67. The method of claim 1 wherein the carbon-silica product is pelletized.
 68. The method of claim 1 wherein the carbon-silica product has an internal void volume of about 30% to about 90%.
 69. The method of claim 68 wherein the carbon-silica product has an internal void volume of about 30% to about 80%.
 70. The method of claim 1 wherein the carbon-silica product has an internal void volume greater than about 50%.
 71. The method of claim 70 wherein the carbon-silica product has an internal void volume greater than about 75%.
 72. The method of claim 1 wherein the mixture is reacted in a batch process.
 73. The method of claim 1 wherein the mixture is reacted in a continuous process.
 74. The method of claim 1 wherein the mixture is reacted in a plug flow process.
 75. The method of claim 1 wherein a powder is made from the carbon-silica product.
 76. The method of claim 75 wherein the powder is pelletized.
 77. The method of claim 1 wherein the plant matter and acid solution are contacted in multiple stages.
 78. The method of claim 1 comprising the step of devolatilizing the carbon-silica product.
 79. The method of claim 75 comprising the step of devolatilizing the resultant powder.
 80. The method of claim 79 comprising the step of pelletizing the devolatilized powder.
 81. The method of claim 76 comprising the step of devolatilizing the pelletized powder.
 82. The method of claim 78 wherein the devolatilization step is done by chemical devolatilization, thermal devolatilization, or a combination thereof.
 83. The method of claim 79 wherein the devolatilization step is done by chemical devolatilization, thermal devolatilization, or a combination thereof.
 84. The method of claim 81 wherein the devolatilization step is done by chemical devolatilization, thermal devolatilization, or a combination thereof.
 85. The method of claim 1 wherein the carbon-silica product is further reacted in a carbothermal reactor at a temperature ranging from about 1250° C. to about 2200° C. to produce a silicon-containing product.
 86. The method of claim 85 wherein the carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1300° C. to about 1475° C. to produce a silicon-containing product.
 87. The method of claim 85 wherein the carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1475° C. to about 1600° C. to produce a silicon-containing product.
 88. The method of claim 85 wherein the carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1600° C. to about 2200° C. to produce a silicon-containing product.
 89. The method of claim 78 wherein the devolatilized carbon-silica product is further reacted in a carbothermal reactor at a temperature ranging from about 1250° C. to about 2200° C. to produce a silicon-containing product.
 90. The method of claim 89 wherein the devolatilized carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1300° C. to about 1475° C. to produce a silicon-containing product.
 91. The method of claim 89 wherein the devolatilized carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1475° C. to about 1600° C. to produce a silicon-containing product.
 92. The method of claim 89 wherein the devolatilized carbon-silica product is reacted in a carbothermal reactor at a temperature ranging from about 1600° C. to about 2200° C. to produce a silicon-containing product.
 93. The method of claim 85 wherein the carbothermal reactor is a static bed reactor.
 94. The method of claim 85 wherein the carbothermal reactor is a moving bed reactor.
 95. The method of claim 85 wherein the carbothermal reactor is a rotary tube reactor.
 96. The method of claim 85 wherein the carbothermal reactor is a fluidized bed reactor.
 97. The method of claim 85 wherein the carbothermal reactor is a furnace reactor that employs an external heat source.
 98. The method of claim 85 wherein the carbothermal reactor is a flame reactor.
 99. The method of claim 85 wherein the carbothermal reactor is a laser reactor.
 100. The method of claim 85 wherein the carbothermal reactor is a plasma reactor.
 101. The method of claim 85 wherein the carbothermal reactor comprises an atmosphere containing inert gas.
 102. The method of claim 85 wherein the carbothermal reactor comprises an atmosphere containing nitrogen.
 103. The method of claim 85 wherein the carbothermal reactor comprises an atmosphere containing nitrogen and hydrogen.
 104. The method of claim 85 wherein the carbothermal reactor comprises an atmosphere containing nitrogen and ammonia.
 105. The method of claim 85 wherein the carbothermal reactor comprises an atmosphere containing chlorine. 106-160. (canceled) 